1. Field of the Invention
The invention relates to an optical testing apparatus for an optical system, in particular a depth of field testing apparatus including a plurality of axially spaced markers for testing the depth of field of photographic or video lens systems.
2. Description of the Related Art
A known apparatus for photographically testing a camera lens comprises a flat (two-dimensional) test or resolution or focusing chart which is placed at the plane of focus and photographed, viewed on a video screen, or otherwise used in conjunction with any optical viewing system. Such charts commonly comprise sharply printed characters, lines or patterns in white printed on black or vice versa. One such pattern comprises groups of lines and spaces of known width and spacing representing discrete spatial frequencies. These are often known as "USAF", "NBS" or "Cobb resolution charts". Various aspects of optical performance can be tested using this apparatus. Known tests are described in, for example, Society of Motion Picture & Television Engines (SMPTE) Recommended Practice, March 1979, Volume 88 RP82-1978, pp 172-174, and Method of Determining the Resolving Power of Lenses for Camera (British Standard) BS 1613:1961.
It is also desirable, however, to test the "depth of field" of an optical system such as a camera and lens combination, that is, to test the extent to which nearer and further objects are in and out of acceptable focus. This is because lenses do not only image flat surfaces but they also include objects nearer and further than the plane of focus, such objects being less "sharply" defined. Various parameters influence the depth of field, for example the mathematical co-relationship of the lens focal length, the lens aperture, the focus distance and the aberrational "circle of confusion" at the image plane. The circle of confusion may be defined as the level of blurring of an infinitely small spot of light imaged by the optical system which gives rise to a blur circle at the image plane. For professional 35 mm cinematography a circle of confusion of 1/1000 of an inch (0.0254 mm) is common. A further parameter influencing the depth of field is set by the optical aberrations of the lens or lenses in the optical system. A further discussion of these parameters can be found in "Optical System Design" by Rudolph Kingslake, Academic Press 1983.
In the past, depth of field has been tested by means of what is known as a "harp" chart. This test method uses a rectangular frame with parallel strings or wires drawn from the top of the frame to the bottom. The frame, with the strings set vertically (or horizontally), is then placed obliquely through the plane of focus and in this manner the fall off in focus on either side of the plane of focus can be seen. Tags can be attached to the strings to mark different distances from the optical system being tested.
A problem associated with such an arrangement is that it is imprecise and conveys little quantitative information to the tester.
Further known tests are described for example in "Lens Evaluation with Basic Laboratory Equipment" by Bruce H Walker, Optical Spectra July/August 1970.